My mama didn't hate them, but I never knew much about diesels. During the late 70s fuel scare, one of my many bosses bought an Olds diesel, probably with the 350cc engine, to try to get better economy without buying a small car. He complained about it constantly, and the 350 is now considered one of the worst engines of all time. I drove my aunt's big Mercedes turbo-diesel a few times, but never, ever considered buying a diesel myself. But diesel keeps cropping up in articles, and clean diesels regularly figure in green car competitions. If you've got a pile of cash, you can buy the world green car of 2012, the Mercedes Benz S 250 CDI Blue Efficiency (below) for under $70,000, except that it doesn't seem to be sold in the US.

Diesel has always been popular in Europe, and TTAC has cited reports that diesel sales are up 27%, from a miniscule 0.4% to 0.5% of US sales, and beginning to make some inroads in Japan.

Even with higher taxes and low-sulfur environmental regulations, diesel used to be competitive with gasoline, but something has driven diesel prices consistently higher than gasoline since about 2004. Diesel engines convert about 45% of the energy into work while  gasoline engines only convert about 30%—though with recent efforts that may be changing. And good diesel engines can last twice as long as gasoline engines.

But PetroDiesel fuel—made from petroleum—is nasty stuff. It sticks to your hands, face and clothing rather than evaporating. Its high particulate exhaust is difficult to trap and carcinogenic. But for every guy raving about his hybrid's fuel economy, there's a guy saying, "I've been getting close to that on my VeeDub diesel for decades."

To avoid the toxic exhaust, one can look for B100, 100% cleaner biodiesel, but you'll need a specially-modified diesel engine, and a map or a website to find suppliers. There is also B20, which is only 20% biodiesel, which doesn't sound like much of an improvement, and B5 and B2, which sound negligible, but are approved for most unmodified diesels.

As seen in the 30 Days episode about the ecovillage, one can also modify a diesel to use heated vegetable oil. Vegetable oil is not biodiesel, and biodiesel is not vegetable oil. Biodiesel may be made from vegetable oil, but vegetable oil fuel is actually frowned upon by the feds. Drivers have been harassed for using vegetable oil, and the feds have tried to collect taxes on free waste oil that some use.

But it's kind of amazing to think that waste fryer oil will run your car. If you can find a willing restaurant manager, you might get their waste vegetable oil for free. Conversions use either prefiltered straight vegetable oil (SVO) or waste vegetable oil (WVO) which will make your exhaust smell like french fries. Vegetable oil isn't perfect but it contains no sulfur and is carbon neutral. Opinions vary but CO, CO2, HC and TPM (total particulate matter) emissions seem to be lower. NOx (nitric oxide and nitrogen dioxide) seem to be higher but can be brought to baseline with timing adjustments.

A conversion will cost a thousand dollars or two, though, and the vegetable oil tank will take up space in your trunk, and with WVO you will have to invest time collecting and filtering the oil. You can buy converted cars from specialists like Frybrid or Greasecar. Greasecar advises using pure oil but Frybrid says hydrogenated or partially hydrogenated vegetable oil (HVO) is also fine.

Green Car Congress just reported that researchers have gotten reduced emissions with HVO:

A team of researchers in Finland reports that by adjusting engine parameters for the use of hydrogenated vegetable oil (HVO) renewable diesel fuel across a range of loads (50%, 75%, and 100%), particulate matter and NOx emissions can both be reduced by more than 25% relative to the values from using HVO with standard engine conditions.

I suspect that ICE engines in general are going to get a lot more efficient, and maybe cleaner, from better tuning and start-stop technology, but as with hybrids, the general trend is towards more complexity, thus greater cost to achieve the utility we used to take for granted.